Usually you have 1 or 2 weeks to do you DD but during this time the REA will still keep advertising the property. So there is no real loss.
If it was subject to the sale of another property a vendor would not likely agree unless it has been a really quiet market and/or it was a fantastic...
What formal approval are you waiting on?
To me it seems like he is wanting your offer in writing so he can take your offer to the vendor and/or has another offer and is going to take both offers to the vendor to consider.
If you don't put an offer in and there is another offer when he goes...
